The following method is known: a method in which selenium is treated in such a manner that selenium-containing water such as coal gasification wastewater is contacted with a mixture of metallic titanium particles and metallic aluminum particles, aluminum is eluted by lowering pH during contacting, and selenium in wastewater is removed by subjecting selenium therein to reduction treatment by local cell action (Patent Documents 1 and 2).
The following method has been proposed for treating coal gasification wastewater by comprehensively treating SS, fluoride, cyanide, ammonia, and COD components contained in wastewater together with selenium: a method in which fluorine in wastewater is removed by flocculation-settling treatment, cyanides are decomposed by wet oxidation or the like, COD components and ammonia are oxidatively decomposed by catalytic wet oxidation, and selenium is then subjected to reduction treatment by the above method (Patent Document 3).
[Patent Document 1] Japanese Patent Publication 2008-30020A
[Patent Document 2] Japanese Patent Publication 2009-11915A
[Patent Document 3] Japanese Patent Publication 2010-221151A
Studies performed by the inventors have revealed a problem that selenium-treating performance decreases with time while such a series of treatments as disclosed in Patent Document 3 are being performed. The investigation of the cause of the problem has suggested that noble metal ions are eluted from a noble metal catalyst used for the catalytic wet oxidation treatment of COD components and ammonia prior to the reduction treatment of selenium to act as an inhibitory agent.
Regarding this, Patent Document 1, which relates to the reduction treatment of selenium, discloses nothing about a decrease in treatment performance due to a coexisting substance in selenium-containing water. Patent Document 2 discloses that performance is recovered by activating metallic titanium with acid; however, a technique disclosed in Patent Document 2 is incapable of solving the above problem.
In Patent Document 3, a problem with the elution of the noble metal ions from the noble metal catalyst is not investigated.